Expulsion-type excess-voltage protective tube



O. 'ACKERMANN Filed Sept. 30, 1939 '[nsulafiom INVENTOR Otto Aekermamn.

WITNESSES:

ATTORNEY Patented July 1, 1941 1 UNITED STATES] PATENT OFFICE Otto Ackermann,

Irwin, Pa, assirnom'e Westinghouse Electric & Manufacturing Company, East Pittsburgh, Par, a corporation or Pennsyl- Application September- 30, 193s, sag; ltd-291,337

' 70laims; '(c 17 5----30) l\ My invention relates to excess-voltage protective devices or lightning-arresters oi the expulsion-tube type, in which an excess-voltage dis charge is produced on the inside of an insulating f tube under conditions producing a blast of de- 1 ionizing gases or particles which assist in extinguishing the resulting arc.

The-principal object of my lnventrn is to produce such a tube, which will embody a major improvement in the lightning protective charoperational the device. as a result or this precaution, the tube has a preferential internal acteristic, aswell as a greatly increased range tube i, whi fl is preferably of fibre or other gasevolving insulating material which hasthe propertyv oi; giving ofl large volumes of gases or otherdeionizing particles in the presence or an electric arc. I provide two tubular conducting electrodes 2 and 3 which extend partway into the tube I, from opposite ends thereof, said electrodes being normally insulated from each other, The electrode. 2 is open, at its end,.so as to vent the'interior oi the tube, while the electrode 3' is not only closed by an end-cap l, but also closed by an inner flbre rod-like filler or plug 5, one end of which is tightly pressed into the electrode 3 and riveted in place therein. The thickness the wall of the tubular electrode 3 thus causes the inwardly projecting end oi the fibre plug 5 to be spaced from the bore of the insulating tube 8' by a long, narrow. annular discharge-space or gap which extends from the closed tubular electrode 3 substantially. to the open tubular electrode 2.

Closely surrounding the central 13011310110! the insulating tube i is a strong steel or other metal tube I which terminates short of each end of the tube I, and which is insulated from both of the electrodes 2 and 3. The metal tube 1 is long enough to overlap a portion. 8 of each of the electrodes 2 and 3, with the wall of the insulating" tube I interposed in between.

An adequate external sparking-distance is provided between the respective ends of the flashover ,or discharge path'in the long, narrow,

annular discharge-space 8 between the two elec-- trodes 2 and I within the bore of the insulating.

tube i. In order that said external sparking,- distancermay' be as small as possible, thereby conserving the space required by the protector tube, as well as reducing its cost,-I prefer to fill these external sparking-distances with a liquid or semi-liquid insulating material, such as oil or petrolatum jelly 8, which is confined by means of short insulating tubes Ii, so that the respective ends of the metal tube I are substantially embedded in insulating material, thus increasing the break-down strength of the space surrounding these ends of the metal tube I. 4

- The purpose oi. the metal tube or cylinder I is threei'old. In the first place, it reinforces the fibre tube i and prevents its bursting at any part which is under radial stress during the discharge-conditions. V

In the second place, the metal tube 1, by reason or its overlapping with the electrodes 2 and 3 produces a strong electrostatic coupling between the metal tube or shield I and each of the electrodes 2 and 8, each oi. these electrostatic couplings being of something like the same order of magnitude, and being considerably largerthan the electrostatic coupling between the shield I,

and any reasonably spaced extraneous object, a

such as a grounded portion oi'the supportingtower (not shown), whereby a material prop rtion of the total inter-electrode potential-difference is impressed at each of said electrostaticcouplings between the shield i and the respective electrodes 2 and 3. If the overlapping distance isthe same for eachoi'the respective electrodes, and it the intervening wall-thickness oi the insulating tube is the same for each or these overlapping portions 8, the two aforesaid v electrostatic couplings will be identical, so that- I exactly half of the voltage-distance will appear at each coupling, assuming thatthe shield-to ground-capccitance'is relatively negligibly small.

In other words, the metal tube orshield I' willalways have a potential approximately midwaybetweenthe potentials of the two insulated elecmetal tube 1 and the respective metal end-caps .4 and I of the tube, so that these external spark lug-distances will not break down during he H trodes 2 and 3, whether theyare discharging or not. This shielding-arrangement is thus distinctive in being substantially independent oi the capacity of the shield to ground, which was considered to be of essential importance in previous shielding-arrangements which'have been utilized on expulsion-type tubes heretofore.

A third purpose of the metal tube or cylinder I V is to concentrate the total available dischargevoltage near the inner ends of the respective electrodes 2 and 3, which constitute the terminals of the spark-gap or discharge between the two electrodes within the tube I. Since the elec trostatic couplings between the metal cylinder or shield I and the respective electrodes 2 and 3 are substantially equal, or at least of the same order of magnitude, a'half, or some other substantial part, of the excess-voltage surge appears across the thickness of the fibre tube i at the inner end of each of the electrodes 2. and 3, thus creating a strong electrostatic stress at the arcing-tips of the respective electrodes 2 and 3, thus greatly facilitating the initiation of ionization at these inner electrode-tips. Because of the dielectric strength of the fibre-tube i, this ioni-' zation does not produce a discharge -or breakdown'through the wall of the fibre tube, from the electrode to the shield I, but it creeps along the inner bore of the fibre tube I and causes an as possible, so as to transmit practically all of the internal bursting-pressure from the fibre tube l to the steel cylinder 1.

In order to assist in the provision of the aioresaid uniform stress-distribution, and in the transfer of the bursting stresses to the steel cylinder, it is also desirable that, for a given wall thickness, the insulating tube 5 should be of as largea diameter as feasible, so as to have a small ratio of outer diameter divided by inner diameter, because, then, the fibre wall, acting more or It will be typical, with my new type of protector, that the wall-thickness of the fibre tube '5 will increase with the voltage-rating of the tube. In order to-avoid the utilization of insulating fibre tubes having thick walls, such as would reduce the electrostatic coupling and the voltage-stress at the arcing-tips of the respecthe electrodes 2 and 3, it might be desirable, in the protection of lines or apparatus havingvery high voltages, to connect two or more of my tubes in series. The close electrostatic couplings between my shield I and the respective electrodes 2 and 3 serve as a voltage-dividing less like a lining on the inside of a steel tube.

will be under radial compression rather than peripheral tension-stress. A small-bore fibre tube l, with the same wall-thickness, would not so readily transfer its bursting stresses to the outer metal tube i. It is thus desirable to have the inner bore of the fibre tube i so large that it would not provide'a suiiiciently narrow or restricted slot, discharge-space or channel between the two electrodes 2 and 3 if it were not for the provision of an insulating filler of some kind, such as the plug 5, between the electrodes, for providing a slotted passage or passages, 'of sufficiently restricted width, between the inner ends of the respective electrodes 2 and 3. p

When I state that the wall of the fibre insulating tube i should be as thin as possible, I meanthat said wall should have the minimum thickness necessary to provide the dielectric puncture-strength required towithstand half of the fiashover discharge-voltage of the protective tube, with a reasonable factor of safety. In practical designs, it is frequently desirable to provide a suiilcient factor of safety so that the puncture-strength of the fibre wall, between the 7 metaltube I and either one of the electrodes means, so that not only each tube, but each electrode of each tube, gets its own proper share,

or divided part, of the total overallvoltage. I can thus construct my improved protector tubes for a reasonable voltage of some low or intermediate value or values, and I may utilize a plurality of tubes connected in series with each other forthe higher voltage-ratings.

While I have described and illustrated my invention in a single illustrative form of embodiment, which is at present preferred by me, I desire it to be understood that such description and illustration are to be understood in an illustrating sense rather than a limiting sense, at least insofar as the broader principles of my invention are concerned, as I realize that many changes, by way of additions, omissions, alterations, and substitutions may be made by those skilled in the art without altogether departing from some of the broader aspects of my invention. I desire, therefore, that the appended claims shall be accorded the broadest constructitotn consistent with their language and the prior a I claim as my invention:

1. An expulsion-type excess-voltage protective tube comprising a thin-walled insulating tube. a conducting electrode extending partway into the tube at each end thereof, said electrodes being normally insulated from each other, at least one of the aforesaid parts being provided with venting means, a conducting strengthening-tube insulated from both electrodes and surrounding the central portion of said insulating tube to augment the bursting-strength of the latter, said conducting strengthening-tube terminating short of both ends of the insulating tube and overlapping a predetermined portion of each of the electrodes, with the wall of the insulating tube interposed in between, said predetermined overlapping portion being of substantially the same distance for each electrode, at least said interposed wall-portions of the insulating tube being of substantially the same thickness for each electrode, which thickness is substantially the minimum thickness necessary to provide the dielectric puncturestrength,required to withstand half of the flashovendischargewoltage of the protective tube, with a reasonable factor of safety, the inner bore of the insulating tube being larger than in desirable for extinguishing the internal arc be tween the inner ends of the electrodes, and an inner plug of insulating material partly filling the bore of the insulating tube between the electrodes so as to provide a restricted-width slotted passage or passages between the inner ends of the electrodes, the wall-material of said slotted passage or passages including gas-evolving areresponsive material, and the construction being such that a flashover always occurs internally within the insulating tube between the inner nds of the electrodes.

- 2. An expulsion-type excess-voltage "protective tube comprising an insulating tube, a conducting electrode extending partway into. the

, tube at each end thereof,.'said electrodes being normally insulated from each pthen'at least one of the aforesaid-parts being provided with ventlng means, a tubular shield insulated from-both electrodes. and surrounding the central portion of said insulating tube and terminating short of both ends of the insulating tube and overlapping a predetermined portion of each of the electrodes,"

with the wall of" the insulatingtube interposed in between, said predetermined overlapping portionbeing of substantially the same distance for each electrode, atleast said interposed wall-portions of the insulating tube being of substantially tions of 'the insulating tube beingotsubstantially the same thickness for each electrode,

which thickness issubstantially the minimum thickness necessary to provide the dielectric puncture-strength required to withstand half of the same thickness for eachelectrode, the cone struction being such as to provides. restrictedwidth slotted passage or passages between the inner ends of the electrodes, the wall-material of said slotted passage ormpassagesaincluding gas-Waving r -responsive material; and th K construction being such that. a fl ashover;always occurs'internallywithinthe insulating tube between the inner-ends of he ele trodesq 3. An expulsionetype excess v'otage protective tube comprising an insulating-tube, a conducta ing electrode extending partway into the tube at L 1 each end .thereoLsaid electrodes fbeing normally insulated from each other, .a'taleast one ofthe aforesaid. parts beingv providedfwith venting means, a conducting strengthening-tube insue lated from both electrodes and-surrounding the central portion :of said insulating tube to'augment the bursting-strength of the latter, said the flashover discharge-voltage of the protective tube, with a reasonable factor of safety, the inner bore of the insulating tube being the same for the twointerposed portions between the conducting strengthening-tube and the inner ends of the respective electrodes and being ;-larger than is desirable for extinguishing the internal "arc" .between' the inner ends of the electrodes, and an insulating filler between the electrodes for providing a restricted-width .slotted passage or passages between the inner ends of the electrodes, the'wall-material of said slotted passage or passages including gas-evolving arc-responsive'ma-. terial, and the construction being such that a .flashover always occurs internally within the insulatingi tube between theinner'ends of the 5. An expulsion-type excess-voltage protective' tube comprising an insulating tube, a conducting -eIectrode extending partway into-the tube at" each end thereof, said electrodes being normally insulated from each other, at least one of the aforesaid parts being, provided with .venthig means, a tubular shield insulated 1mm both elecsaid ins-mating tube and terminating." short of' both ends of the insulating tube and overlapping.

' a predetermined portion 'of each' of the. elec conducting strengthening-tube terminating short of both ends of-the insulating tube and over-' lapping a predetermined portion of each of the electrodes, with the wall of the insulating tube interposed in between, said predetermined overlappingportion being of substantially the same distance for each electrode, .at least said'interposed wall-portions' of the insulating tube bein of substantially thesa'me. thickness for each electrode, which thickness is substantially thetminipling between the shield and any reasonably.

trodes and surrounding the central portion of trades, with the wall oi the insulating tube in-' terposed in'between, the electrostatic coupling between the shield and each electrode being of something like the same order of magnitude, and

considerably larger than-the electrostatic couspaced extraneous object so that the potential of the shield is substantially altogether determined .by the potentials of the electrodes independently main thickness necessary to provldethedielew" tric puncture-strength required to withstand half of the'fiashover discharge-voltage of the protective tube, with a reasonable factor of safety, the construction being such as to provide a restrictor the position of the protective-tube with re-'. spect toany extraneous object, whereby a predetermined material proportion of the totalinterelectrode potential-difference is impressed. at each of said electrostatic couplings between the ed-width slotted pais'sagefor passages between-the 1 inner ends of the electrodes, the wall-material of saidslotted passage or passages includinggas- 5 evolving arc-responsive material, and the: construction being such that a 'flashover always. oc-

'curs internallywithin the insulating tube between t the inner ends of the electrodes;

Y 4. Anf-expulsion-type excessvoltage protective tube comprising an insulating tube, a conducting electrode, extending partway into the tube at each end thereof, said-.electrodesbeing normally-insulated from each othergat least one of the afore said parts being provided with venting means, a conducting strengthening-tube insulated from both electrodes andsurrounding the central portion of said insulating tube to augment the bursting-strength .of -the latter, said conducting strengthening-tube terminatin short of both ends of the insulating 'tube and. overlapping apredetermined portion of each of the a egctrodes, with the wall of. the insulating tube inte used in betw een, .said D 'F 'etermined overlapping porshield and the respective 'electrodes,' the con- .struction being .such as to provide a restrictedwidth slotted passage or passages between the, inner ends of the electrodes, the wall-material of said slotted passage or passages including gas-:1 evolving arc-responsive material, and. the 6011-.

struction being such that a flashover alwaysoccurs' internally within the insulating tube bee, tween the inner'ends of the electrodes.

6. The invention as specified in claim 1, char;

acterized by each of electrodes being tu bular, one of said tubu e! trodesbeingopem .and said inner insulating.-.-pl ug being: within the other tubularelctrode so as to provide an 'annular discharge-space for the dis-- charge between-theinner ends of the electrodes.--

7. The invention as-specifled in claim 5, characterized by means for embedding the respective endsvofthe tubular shield in insulating material as an aid to preventing an external breakdown .or fiashover of the protective tube.

'1 o'n'o ACKERMANN.

. 'tion'being of substantially the same distance for each electrode, atleast said interposed wall-por- 

